1. Field of the Invention
The present invention relates to an exhaust gas purification system used in an exhaust system of an internal combustion engine for purifying exhaust emissions.
2. Description of the Related Art
It has conventionally been proposed, as a technique for purifying exhaust emissions from an engine, to use an exhaust gas purification system that uses plasma generated by an ac voltage applied across electrodes (a plasma reactor) For example, a system disclosed in Japanese Patent Application Laid-open No. 7-293223(1995), is provided with a counter electrode disposed in an exhaust emission flow path from an engine and a discharge electrode disposed upstream of the counter electrode. Another system disclosed in Japanese Patent Application Laid-open No. 7-265652(1995) includes a honeycomb structural member used as a counter electrode disposed in an exhaust emission flow path from an engine and a discharge electrode disposed so as to run through a cell of the honeycomb structural member.
In these systems, when a high voltage is applied across the discharge electrode and the counter electrode, energy of plasma thereby generated sets HC, CO, NOx, and other substances contained in exhaust emissions in a free state (free radicals). These substances are then purified through chemical reactions with oxygen contained in the exhaust emissions.
In such systems, particulate matter, or PM, charged by the discharge electrode, is attracted by and adheres to the counter electrode of an opposite polarity. PM may densely adhere to some local areas of the counter electrode, while sparingly at other areas. If this happens, densely populated PM on a narrow area tends to become incompletely combustible. Then, residual ashes left unburned contribute to a reduced cross sectional area of the exhaust emissions flow path (an opening area). This results in an increase in pressure loss and a plugged up flow path, thus degrading purification performance of the system.
It is therefore an object of the present invention to provide means for suppressing uneven, or partly dense and partly sparing, adherence of PM on the counter electrode.